This invention relates to microelectromechanical structures and more specifically, to electrostatically actuated mirrors in such structures, and particularly to electrodes of the electrostatic actuators.
Microelectromechanical structures (MEMS) and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Many different varieties of MEMS devices have been created, including microgears, micromotors, and other micromachined devices that are capable of motion or applying force. These MEMS devices can be employed in a variety of applications including hydraulic applications in which MEMS pumps or valves are utilized and optical applications which include MEMS light valves and shutters.
MEMS devices have relied upon various techniques to provide the force necessary to cause the desired motion within these microstructures. Some MEMS devices are driven by electromagnetic fields, while other micromachined structures are activated by piezoelectric or electrostatic forces. MEMS devices that are actuated by the controlled thermal expansion of an actuator or other MEMS component have also been developed.
MEMS devices including moveable mirror structures have also been developed. Commonly, MEMS moveable mirror devices have been used to redirect electromagnetic energy traveling along a path, typically a light or laser beam. For instance, U.S. patent application Ser. No. 08/719,711, incorporated by reference herein, describes various types of MEMS devices which can rotate a reflective plate about several axes within a framed structure.
As shown in FIGS. 1a and 1b, explained in detail below, a known electrostatically actuated MEMS mirror structure has a mirror crystal mounted above a substrate for tilting about an axis which is parallel to the substrate. Two stationary electrodes are mounted on the substrate at locations corresponding to the ends of the mirror such that when electrostatic charge is applied to one of the electrodes, the resulting electrostatic force pulls the corresponding end of the mirror towards the electrode to effect a tilting movement of the mirror. The amount of the movement can be controlled such that the tilting progresses up to the point when the end of the mirror touches the electrode.
It is known that in electrostatic actuation, the force generated between the electrode and the tilting mirror element (e.g. a crystal plate) is inversely proportional to the square of a gap between the electrode and the tilting element. Also, the actuation mechanism in the arrangement such as shown in FIGS. 1a and 1b becomes non-linear after the moving plate (mirror) traverses about ⅓ of the initial gap between the electrode and the plate, it has been attempted to devise structures where the driving mechanism (electrodes) would be non-parallel to the moving plate. However, the commercial-scale design of such a structure has encountered problems, mostly because of the difficulty in etching semiconductor structures at an angle to the main plane.
The invention attempts to provide a solution to the above-described problem.
The invention stems from a finding that an improved MEMS mirror structure, with electrodes at an angle conforming to an angle of the mirror when tilted, can be provided by providing a deformable electrode support and deforming means mounted for permanent predetermined deformation of the electrode support with electrodes thereon. The deforming means may be provided in the form of a deforming member with one or more protrusions disposed to deform the electrode support in a predetermined manner, or a deformable electrode support with one or more protrusions and a complementary deforming member, all the elements shaped and disposed to effect, when assembled, a predetermined deformation of the electrode support and the desired positioning of the electrodes thereon.
In accordance with the invention, there is provided a movable microelectromechanical mirror structure comprising:
a microelectronic substrate defining a first major surface;
a mirror having two ends and disposed for tilting movement in response to an attractive force about an axis disposed out of plane relative to the first major surface, the tilting movement varying between a non-actuated position and a fully actuated position at an end point of the tilting movement, the fully actuating position resulting in an angular position of the mirror at a limit angle relative to the major surface, and
a pair of electrodes for exerting each an attractive force on a corresponding end of the mirror, each electrode mounted on the substrate at a position corresponding to the corresponding end of the mirror at an electrode angle to the first major surface, the electrode angle being similar or identical as the limit angle of the corresponding end of the mirror, the dimensions of the mirror, of the electrodes and a spacing between the mirror and the electrodes selected such that the electrode and the corresponding end of the mirror are in an approximately parallel relationship in the fully actuated position when the corresponding end of the mirror becomes adjacent to the electrode due to the attractive force.
The movable mirror structure may comprise a deformable electrode support member mounted on the substrate, with at least one of the electrodes mounted on the electrode support member, and a deforming element mounted against the deformable electrode support member such as to permanently maintain the deformable electrode support member in a deformed state in which the at least one of the electrodes is disposed at the electrode angle.